Sociologist Robert Bartholomew, author of several books on mass hysteria including The Martians Have Landed: A History of Media-Driven Panics and Hoaxes, explained to Discovery News that "there are two main types of contagious conversion disorder. The most common in Western countries is triggered by extreme, sudden stress; usually a bad smell. Symptoms typically include dizziness, headaches, fainting and over-breathing, and resolve within about a day."

In contrast, Bartholomew said, "The LeRoy students are experiencing the rarer, more serious type affecting muscle motor function and commonly involves twitching, shaking, facial tics, difficulty communicating and trance states. Symptoms appear slowly over weeks or months under exposure to longstanding stress, and typically take weeks or months to subside."

Mass hysteria cases are more common than people realize and have been reported all over the world for centuries. Here's a look at some famous - and bizarre - cases of mass hysteria in history.

The Mad Gasser of Mattoon

Many cases of mass hysteria are spawned by reports of strange or mysterious odors. One of the most famous cases occurred in 1944 when residents of Mattoon, Ill., reported that a "mad gasser" was loose in the small town.

It began with one woman named Aline Kearney, who smelled something odd outside her window. Soon she said her throat and lips were burning, and she began to panic when she felt her legs becoming paralyzed. She called police, and her symptoms soon subsided. Her husband, upon returning home later, reported glimpsing a shadowy figure lurking nearby. The "gas attack" (as it was assumed to be) on Mrs. Kearney was not only the gossip of the neighborhood but also reported in the local newspaper, and soon others in the small town reported odd odors and experiencing short-lived symptoms such as breathlessness, nausea, headache, dizziness and weakness. No "mad gasser" was ever found, and no trace of the mysterious gas was detected.

The French Meowing Nuns

Before 1900 many reports of mass hysteria occurred within the context of religious institutions. European convents in particular were often the settings for outbreaks. In one case the symptoms manifested in strange collective behavior; a source from 1844 reported that "a nun, in a very large convent in France, began to meow like a cat; shortly afterwards other nuns also meowed.

At last all the nuns meowed together every day at a certain time for several hours together." The meowing went on until neighbors complained and soldiers were called, threatening to whip the nuns until they stopped meowing. During this era, belief in possession (such as by animals or demons, for example) was common, and cats in particular were suspected of being in league with Satan. These outbreaks of animal-like noises and behaviors usually lasted anywhere from a few days to a few months, though some came and went over the course of years.

The Pokémon Panic

A strange and seemingly inexplicable outbreak of bizarre behavior struck Japan in mid-December 1997, when thousands of Japanese schoolchildren experienced frightening seizures after watching an episode of the popular cartoon "Pokémon." Intense flashes of light during the show triggered relatively harmless and brief seizures, nausea, and headaches. Doctors diagnosed some of the children with a rare, pre-existing condition called photosensitive epilepsy, in which bright flashing lights used in the cartoon can trigger the symptoms.

But experts were unable to explain what had happened to the remaining thousands of other children who reported symptoms; the vast majority of them did not have photosensitive epilepsy. Finally, the mystery was solved in 2001, when it was discovered that the symptoms found in most children were caused by mass hysteria, triggered by the initial wave of epileptic seizures.

The McMinnville School Poison Gas Episode

Nearly 200 students and teachers were hospitalized during a mysterious outbreak of illness at Warren County High School in McMinnville, Tenn., in November 1998. A local newspaper, the Southern Standard, ran the headline "Students Poisoned: Mysterious Fumes Sicken Almost 100 at High School." It began when a teacher reported smelling a gasoline-like odor in her classroom that made her sick. A few of her students then also became sick, and the school was closed for testing.

No contamination was found, nor any medical or environmental cause for the symptoms, which included headache, dizziness, nausea and drowsiness. Following a clean bill of health, the school reopened, and soon a second cluster of students fell ill and closed down the school a second time. All recovered from the attack.

As these cases show, the LeRoy high school incident is only one of many strange episodes of mass sociogenic illness - and there will be more.